Angus Maguire, head of modern and contemporary art for Dreweatts and Bloomsbury Auctions, said: "It's a big size and a new and unique image - that makes it incredibly interesting to collectors.

"But more than that you've got this amazing provenance, this story that has already evolved about this particular artwork.

"So there's three factors - a new breed of avid collectors, a unique image and this incredible provenance - and these three factors must produce a spectacular result if this work ever came to auction."

The elated Broad Plain Boys' Club has welcomed the twist with open arms, as only a month ago it was appealing for £120,000 to change its "very bleak" future and the imminent threat of closure.

Mr Stinchcombe, who believes Banksy is a supporter of the 120-year-old club due to his own childhood experiences, claimed he had already received an offer of £1m for the Mobile Lovers piece.

Meanwhile it remains safe and on display at the Bristol Museum and Art Gallery.